mmc: meson-gx: fix __ffsdi2 undefined on arm32

Using __bf_shf does not compile on arm 32 architecture.
This has gone unnoticed till now cause the driver is only used on arm64.

In addition, __bf_shf was already used in the driver without any issue.
It was used on a constant value, so the call was probably optimized
away.

Replace __bf_shf by __ffs fixes the problem

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>